Generally, many organic polymers have low rigidity, low hardness and low thermal resistance. As an attempt to improve these properties, the formation of composite materials by combination with inorganic materials has frequently been investigated until now.
As composite materials containing colloidal silica, for example, coating compositions comprising an aqueous emulsion of an acrylic polymer having alkoxysilyl or hydroxyl groups and colloidal silica are disclosed in Japanese Patent Publication Nos. 41180/'89, 48832/'92 and 40796/'93, Japanese Patent Laid-Open Nos. 287217/'93, 199917/'94 and 26165/'95, and Japanese Patent Publication No. 94620/'95. These coating compositions are effective in modifying the surface properties of plastic moldings, but cannot improve the rigidity, toughness and thermal resistance of the moldings themselves.
Moreover, thermoplastic resins containing a core/shell material having colloidal silica as the core and polyorganosiloxane as the shell, and a vinyl monomer graft-copolymerized thereto are disclosed in Japanese Patent Laid-Open Nos. 223333/'91, 231925/'91, 270710/'92 and 117338/'93. The process disclosed in these patents comprises polymerizing a siloxane in an aqueous dispersion of colloidal silica to prepare a silica core/silicone shell material and then effecting the emulsion polymerization of a vinyl monomer thereon to obtain a vinyl polymer-grafted core/shell material. Consequently, this process has the disadvantage that, unless a high proportion of a silicone component is added, a large amount of undesired solid matter (cullets) may be formed in the polymerization system and that, depending on the type of the vinyl monomer, the components within the polymerization vessel may solidify during polymerization and fail to yield a stable emulsion. That is, this process involves considerable limitations on composition. Moreover, this process allows colloidal silica to assume only a morphology in which it is completely covered with silicone rubber, and fails to achieve an effect anticipated by incorporating colloidal silica in a resin (i.e., an improvement in the rigidity and thermal resistance of molded articles). Actually, it is theoretically and experimentally shown in J. Mat. Sci., Vol. 29, p. 4651 (1994) that high rigidity cannot be achieved in a structure in which an inorganic filler is coated with a rubber component.
Furthermore, Japanese Patent Laid-Open No. 322036/'94 discloses a process for the preparation of a silica-containing acrylic polymer composition which comprises subjecting a (meth)acrylate and a crosslinkable monomer to emulsion polymerization or suspension polymerization and, when a predetermined degree of polymerization is reached, adding fine silica particles that have previously been treated with a silane coupling agent. In this process, however, a crosslinkable monomer is indispensably used in order to disperse silica satisfactorily and to crosslink the polymer thermally by subsequent addition of a crosslinking agent. Moreover, fine particles of silica powder must previously be dispersed in water and treated with a silane coupling agent. Thus, this process inevitably requires a complicated procedure. Furthermore, since the fine particles of silica powder dispersed in the acrylic polymer have a size greater than a submicron order, it is impossible to obtain highly transparent molded articles which are desired in the present invention.
In addition, Japanese Patent Publication No. 50881/'81 and Japanese Patent Laid-Open Nos. 213839/'87, 53727/'95 and 53728/'95 each disclose dispersions and powders of a composite material comprising charged inorganic particles and oppositely charged polymer particles and having a structure in which the polymer particles are coated with the inorganic particles, as well as a process for preparing the same. These patents are directed to soft and sticky polymer particles and are intended to disclose a so-called anti-blocking agent for coating the surfaces of such polymer particles with hard silica and thereby preventing them from sticking together prior to use, thus having applications such as coating materials, adhesives, and toners for copying machines. This process has the disadvantage that, since particles having opposite electric charges are mixed, they tend to agglomerate very rapidly and make it difficult to yield a homogeneous composite material. Moreover, these patents neither disclose nor suggest a melt-formable composite material and a molded article having a substantial thickness and exhibiting high transparency.
Japanese Patent Laid-Open Nos. 175351/'92, 261433/'92 and 88355/'95 each disclose an organic polymer powder carrying thereon an inorganic material derived from an inorganic sol such as colloidal silica and suitable for use as a filler for epoxy resin adhesives. The polymer used therein is likewise a soft and sticky one. Moreover, since emulsion polymerization is always carried out by using a sulfate type emulsifier, it is impossible to coat polymer particles efficiently with colloidal silica remaining in the form of primary particles. In these patents, colloidal silica having a particle diameter of, for example, 10 to 20 nm is mixed with a polymer emulsion in such a proportion as to give a silica content of 10 to 95% by weight, and the resulting mixture is spray-dried to obtain composite particles having a diameter of 0.5 to 10 .mu.m. Since the product is obtained by spray drying, the composition of the product is the same as that of the charged materials. However, this method does not produce composite particles comprising polymer particles coated with colloidal silica remaining in the form of primary particles, but produces composite particles comprising polymer particles carrying thereon silica agglomerates derived from colloidal silica. Moreover, when large particles of radius a are coated with small particles of radius b, the maximum number of coating particles (R) can be calculated according to the following theoretical equation of F. K. Hansen et al. ##EQU1##
Thus, it has been demonstrated that, when colloidal silica having a particle diameter of 10 to 20 nm is used in a content of 10 to 95% by weight, particles having a diameter of 0.5 to 10 .mu.m cannot be coated with colloidal silica remaining in the form of primary particles.
For example, Example 15 of Japanese Patent Laid-Open No. 175351/'92 discloses a composite material comprising polymer particles having a diameter of 0.5 to 4 .mu.m which carry thereon 42.9% by weight of silica derived by using colloidal silica having an average particle diameter of 15 nm as a starting material. When the actual number of colloidal silica particles attached to each polymer particle in this composite material was calculated, it was more than 3.5 times greater than the maximum number of coating particles (R) expressed by the above equation. That is, this powder does not carry thereon an inorganic sol having a size of the order of nanometers in the form of primary particles, but carries thereon agglomerates of an inorganic sol.
Japanese Patent Laid-Open No. 209027/'93 discloses a highly transparent composite material which is obtained by treating the surfaces of colloidal silica having dispersibility in organic solvents or water with an alkoxysilane compound, replacing the dispersion medium by a radical-polymerizable vinyl compound, and then polymerizing the resulting mixture. However, this composite material has a mutually penetrating network structure consisting of an organic polymer and a silica network, and is hence incapable of melt forming. Moreover, Japanese Patent Laid-Open No. 287213/'93, Journal of the Japanese Society of Textile Science, Vol. 49, p. 130 (1993), and Polym. Adv. Tech., Vol. 3, p. 91 (1992) each disclose a colloidal solution obtained by modifying colloidal silica dispersed in an organic solvent with a chain polymer compound, as well as a composite powder obtained by separating the solvent by distillation or centrifugation. However, these processes are unsuitable for industrial purposes because they involve high raw material costs and require troublesome process steps.
Furthermore, composite particles obtained by coagulating an organic polymer latex and silica particles are disclosed in "A Collection of Papers on Polymers", Vol. 46, p. 21 (1989), ibid., Vol. 44, p. 483 (1987), and ibid., Vol. 44, p. 839 (1987). However, large silica particles used in these references, which have a diameter of 240 to 1,590 nm, fail to yield highly transparent molded articles desired in the present invention.